The present invention relates to a central differential provided in a power transmission system for a four-wheel drive motor vehicle.
Japanese Patent Application Laid-Open 55-72420 discloses a central differential which comprises a differential case, encasing two differential pinion gears and two bevel side gears meshed with pinion gears, and internally splined to drive wheel axles, and a ring gear attached to the case and engaged with a drive pinion.
In order to restrain the differential operation, two sets of clutch plates operated by oil pressure are provided for locking the drive wheel axles to the case on a slippery road. Pressure oil is applied to clutches through a hydraulic circuit having a solenoid operated valve. Accordingly, the structure of such a central differential is complicated. Further, the central differential is locked, after a control system detects slipping of wheels in accordance with the speed difference of front and rear drive wheels in excess of a predetermined value. Thus, the locking operation is inevitably ineffective.
Japanese Utility Model Application Laid-Open 61-123250 discloses a differential comprising a worm gear and a worm wheel for permitting the differential operation when a motor vehicle makes a turn. When the vehicle travels on a straight road, the differential is locked because of the irreversible function of the worm gear. This differential may be employed for a central differential.
However, in such a structure, since the worm gear device has a large sliding resistance, the transmission efficiency is reduced and wearing of the gears increases. Further, since the worm gear is used, the structure of the differential is complicated so that manufacturing cost increases.